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The Fabry-Perot Etalon

The interferometer, which the H-alpha filter uses as a basis for its function, is a conglomerate of lenses and mirrors. The source emits light, incident on the interferometer. The first component of the interferometer is a convex lens, which directs the beam to a piece of apparatus, within the interferometer, called the etalon. The etalon, in the filter, is two fixed parallel plates of a transparent medium (i.e. glass) on which is a highly reflective coating (usually aluminium) on the inside surfaces, facing each other. The light beam that enters the first plate is either reflected off the surface, or transmitted through. The ray then goes through an air gap (ranging from nm's to cm's) to the next plate. Once there, the ray will either be transmitted or reflected. If reflected it will be redirected to the first coating and the same event, of transmission or reflection, will occur. However, the distance between the two plates is such that constructive interference will occur with a specific wavelength that coincides with double the distance between the plates. The other wavelengths will act destructively with other reflections and so only the wavelength required is transmitted. This is how the filter works to provide an image in the waveband 656.28 +/- 0.07nm. The transmitted waves, all of which are parallel to each other, exit the etalon and enter another lens which focuses them to a point and hence create the image that we see. The image is actually about two orders of magnitude higher in intensity than when it started out due to the transmission of waves that have acted constructively with each other.

Schematic diagram of the Fabry-Perot interferometer, the interior design of the filter

Schematic diagram of the Fabry-Perot interferometer, the interior design of the filter.


The tuner, therefore, works by altering the path difference in the etalon and so accommodates the shorter wavelengths as the conventional ones now act destructively.